This invention relates to an on-board inert gas generation system. Specifically, the invention relates to a method and apparatus for increasing the pressurized air provided to an air separation module of the system.
On-board inert gas generation systems are increasingly used in aircraft to provide inert gas to various locations such as the cargo area or fuel tanks. Typically, the system includes two compressors or stages arranged in series for providing compressed air to the air separation module. It is desirable to provide high pressures to the air separation module to reduce the weight of the module. Lowering the inlet pressure to the module greatly increases its weight.
Centrifugal compressors are typically used in the system to provide compressed air to the module. It is desirable to ensure that the compressors do not operate in a surge condition, which is a transient phenomenon that occurs when flow through the compressors is reduced below the designed flow rate. Surge can damage or destroy a compressor. Surge can be difficult to avoid since on-board inert gas generation systems typically operate under a wide range of flow rates.
One prior art system includes a nitrogen enriched airflow valve arranged downstream of the air separation module. The nitrogen enriched airflow valve operates by either modulating between a small area position and a range of large area positions or by switching between a large and a small area position to vary the flow through the valve. Prior art systems have also included a surge control valve downstream of the first compressor to selectively vent the compressed air overboard. The surge control valve moves between open and closed positions to increase the flow through the first compressor to keep the compressor from going into a surge condition when the nitrogen enriched airflow valve closes to a smaller area restricting flow through the system.
The nitrogen enriched airflow valve and surge control valve employed separate actuators. If the surge control valve fails when the nitrogen enriched valve is in a small area position then a surge condition will result in the first compressor leading to a mechanical failure of the compressor.
What is needed is a simplified surge protection system that is more reliable in the event of a surge control valve failure.